May I suggest DiY Plasti Dip, cost is like $100 and 4 hours of your time.
I'm doing my rims right now (sealing 22" Onyx black with gloss Plasti Dip to prevent minor scratches), first timer DiY with Plasti Dip.
I know it's not completely the same to what you are looking for but this is another way to get silver to black 20" rims - with fraction of cost.
Before gloss seal: